Mr Rove was also investigated in connection with the exposure of CIA agent Valerie Plame, though prosecutors decided he should not face any charges.

Mr Rove told the Wall Street Journal that he had first floated the idea of leaving last year, but had delayed his departure when the Democrats took control of Congress.

He said he took a final decision to leave after White House Chief of Staff Joshua Bolten told aides that if they stayed after the end of August they would be obliged to stay in the administration until Mr Bush's own departure in January 2009.

"There's always something that can keep you here, and as much as I'd like to be here, I've got to do this for the sake of my family," he said.

He said he expected Mr Bush's current poor ratings to improve, and that conditions in Iraq would get better as the military surge continued.

A Republican had a good chance of winning the 2008 presidential election, he said, because Democrats would choose the "fatally flawed" Hillary Clinton as their candidate.